We're not exactly sure who determines what are electives and what are "core" subjects, or why. Okay, maybe math is more useful than tracing your family tree because you can't balance your checkbook using scions and second cousins. But what about your family history, and the history of, say, Guam or Haiti? Is there something inherently better about reading "the classics" than learning Mandarin Chinese? Why are physics part of the main coursework for high school students, while robotics is optional?
Unfortunately, we aren't qualified to answer those questions, but we can defend electives on another level. Electives are essentially those subjects best suited to a student's personal interests, talents and abilities. In that sense, they're almost more important than core subjects, though we aren't going to make an argument that you should learn French before mastering English grammar (as long as you live in an English-speaking area, that is). We are arguing for advances beyond the necessary "standards," however, and for a broad curriculum that is child-specific.
Some electives are more important than others. Learning to manage one's money, how to interpret the Bible, and (these days) how to operate and understand computers are all more important than taking an acting class (though the acting class will be very important for some on a different level altogether). Technically, none of these are required courses, though for Christians particularly the first two are essential for a proper education. There are plenty of similarly important topics as well, though each family will need to prioritize.
There seems to be a move toward less specialized education. As our society becomes increasingly homogeneous, we spend less time focusing and more time absorbing "the big picture." We ignore either one at our own risk. We must have a larger framework in which to fit the smaller pieces, but take away the framework and you're left with relativity and chaos; the smaller pieces, and you have only an impersonal and meaningless ideology. Electives are a way to assert the need for both, and to have fun doing it.